Polytetrafluoroethylene (hereinafter "PTFE") has excellent heat resistance, chemical resistance, insulative properties, non-adhesiveness and self-lubrication. This polymer has found wide use in medical, industrial and recreational fields.
U.S. Pat. No. 3,953,566 provides a method for producing a wide variety of shaped articles such as films, tubes, rods and continuous filaments. The articles so produced are characterized by a unique combination of high strength and high porosity. The articles, themselves, are covered by U.S. Pat. No. 4,187,390. The microstructure of these articles consists of nodes interconnected by fibrils.
Subsequent work lead to the discovery that expanded PTFE tubes, made according to the teachings of U.S. Pat. No. 3,953,566 in which the distance between nodes ranged from 5-1,000 microns was especially useful as a vascular prosthesis. On implantation in the human body this microstructure is readily invaded by body tissue.
It was also discovered that products could be produced by very high elongation processes, in excess of 50:1. Such products are covered by U.S. Pat. No. 3,962,153. However, studies of the product made by U.S. Pat. No. 3,962,153 at extremely high stretch amounts show that the number of visible nodes were considerably less than the number per unit sample size obtained at lower stretch amounts. The microstructure of these highly stretched samples appeared to be, principally, highly oriented fibrils, oriented parallel to the stretching direction, separated by void spaces, and containing very small and relatively few nodes.
It is thought that the nodes have been largely drawn into fibrils. Although these products are extremely valuable for many applications such as high strength packing fibers and sewing thread, the reduction of the node structure is undesirable for other end uses. The strength of these products is prodominately in the direction of orientation; the strength in the other directions being comparatively weak.
In certain areas it was desirable to have thicker material that was routinely produced by the process of U.S. Pat. No. 3,953,566. A first approach to achieving this would be to start the expansion process with a thicker extrudate. When one considers all the variables involved in paste extrusion, however, it was realized that the extrusion of extrudates with cross-sectional areas greater than about 2 square inches would involve practical extruder designs.
The next step involved the assembly of a multiplicity of thin components to give a thicker component.
A number of prior art methods have been utilized to bond multiple layers of porous PTFE together including:
(1) placing layers of porous PTFE in contact and under pressure and heating above the crystalline melt point of PTFE; PA1 (2) bonding layers of porous PTFE using bonding agents such as silicone adhesives or FEP, a copolymer of tetrafluoropropylene and hexafluoropropylene; PA1 (3) mechanically connecting porous layers using various sewing and/or weaving techniques. PA1 (a) inadequate interlayer bonding; PA1 (b) altered porosity either at the layer interface or within the bulk of the layer; PA1 (c) the presence of contaminating material.
The above described methods suffer from one of more of the following disadvantages:
Specifically in the area of filtration, the microstructure of nodes and fibrils present in products produced by the process of U.S. Pat. No. 3,953,566, and defined in U.S. Pat. No. 4,187,390 is particularly desirable. Interruption of this microstructure of the bond severely affects the utility of the product.